The emerging quantum technological apparatuses(1,2), such as the quantum computer(3-6), call for extreme performance in thermal engineering(7). Cold distant heat sinks are needed for the quantized electric degrees of freedom owing to the increasing packaging density and heat dissipation. Importantly, quantum mechanics sets a fundamental upper limit for the flow of information and heat, which is quantified by the quantum of thermal conductance(8-10). However, the short distance between the heat-exchanging bodies in the previous experiments(11-14) hinders their applicability in quantum technology. Here, we present experimental observations of quantum-limited heat conduction over macroscopic distances extending to a metre. We achieved this improvement of four orders of magnitude in the distance by utilizing microwave photons travelling in superconducting transmission lines. Thus, it seems that quantum-limited heat conduction has no fundamental distance cutoff. This work establishes the integration of normal-metal components into the framework of circuit quantum electrodynamics(15-17), which provides a basis for the superconducting quantum computer(18-21). Especially, our results facilitate remote cooling of nanoelectronic devices using faraway in situ-tunable heat sinks(22,23). Furthermore, quantum-limited heat conduction is important in contemporary thermodynamics(24,25). Here, the long distance may lead to ultimately efficient mesoscopic heat engines with promising practical applications(26).
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